In the past, a variety of methods were used to form semiconductor image sensors on a substrate with complimentary metal oxide semiconductor (CMOS) devices. Typically, the optical receiving portion of the sensor is formed either as a gate of a large area transistor, often referred to as a photo-gate, or as a source-drain junction of a metal oxide semiconductor (MOS) transistor. The photo-gate transistor implementation requires that light travel through the silicon gate of the transistor in order to convert light to electrical energy. Consequently, the photo-gate implementation has reduced sensitivity. Additionally, the depletion region generally is shallow (less than one micron) thereby reducing the collection efficiency of carriers induced by red light absorption. Also, conventional photogate implementations are susceptible to noise created by surface recombination.
The source-drain junction implementation generally has a junction that is optimized for transistor operation and therefore also has a shallow junction that results in efficient collection of carriers induced by red light. Another disadvantage of the source-drain junction implementation is that the junction typically is formed in a highly doped (greater than 10.sup.16 atoms/cm.sup.3) region that limits the width of the junction depletion region thereby further reducing the collection efficiency of carriers induced by red light absorption. Furthermore, forming the junction in such a highly doped region results in a large capacitance that reduces the amount of charge that can be transferred from the photo sensing element producing image lag artifacts. Additionally, such highly doped regions typically have high dark current, leading to high dark noise. Other implants in sub-micron CMOS processes that could be used as a photodiode that are lightly doped are too shallow. Those that are deep enough are too heavily doped.
Accordingly, it is desirable to have an image sensor that does not utilize a photo-gate thereby resulting in higher efficiency, that does not have a shallow junction depth thereby increasing efficiency, that minimizes noise from surface recombination and that has a more lightly doped profile.